Neptune in 2014–’15
2021 October 1
Since the demotion of Pluto to a dwarf planet, Neptune is the outermost planet in the solar system. For amateurs, it is a difficult object to study because of its small diameter of only 2.4 arcseconds. A telescope with a large aperture is required to visually observe the disc of the planet, but detection of its large satellite Triton is relatively easy in medium-sized telescopes due to its brightness of magnitude +13.5. For astrophotographers, recording Neptune and Triton is a challenge. In general, no details on Neptune are visible, except that the southern hemisphere is sometimes slightly brighter than the northern (as shown in Figure 1).
The visit of Voyager 2 to Neptune in 1989 brought a major breakthrough in our understanding of the planet. This spacecraft discovered dark and bright storms, which are sometimes very stable. It also became clear that there are very rapid atmospheric motions. In the Equatorial Zone atmospheric streams rage continuously at a speed of 1,200km/h or more, and the rotation period decreases towards the pole.
Neptune in 2014
In 2014 Neptune was located in Aquarius and was at opposition on Aug 23. The number of Neptune observers in 2014 was very limited and the observations submitted were scarce. A list of the observers, their locations and their instruments is presented in Table 1.
Most observers show Neptune as a tiny bluish disc without details, with the major satellite Triton as a bright dot (Figures 1 & 2). However, on 2014 Oct 7, Anthony Wesley reported the presence of a bright spot using a 610nm longpass filter. Using the WinJUPOS program,1 the coordinates of the spot were determined. It was located at longitude 138°, latitude 42°S and was shown to rotate with the Neptune globe (Figure 3). Measurements on a tiny disc are rather difficult and errors of ±5° and more occur easily. Since observations of the bright spot were very limited in number, no further analysis of its development could be performed.
Neptune in 2015
In 2015, Neptune was still located in Aquarius and was at opposition on Sep 1. An interim report of the 2015 apparition was published previously.2 Fortunately, the number of observers submitting images of Neptune was much higher in 2015 (Table 2). This was mainly due to an initiative of the professional planetary astronomer Ricardo Hueso Alonso. On 2015 Jul 13, Hueso and his colleagues at Escuela Técnica Superior de Ingeniería in Bilbao, Spain discovered a bright spot at latitude 41°S with the 2.2-metre telescope of the Calar Alto Observatory.3 The spot was named Spot A. Later, some minor bright spots were also detected.3 Alonso invited amateur astronomers around the globe with larger amateur telescopes to investigate whether the bright spot could be detected with their instruments.
On 2015 Jul 20, Sussenbach detected a bright spot on Neptune in the vicinity of the predicted position at the central meridian.4 Comparison of the images of 01:11 UT and 01:59 UT shows that the feature moved with the planet’s rotation (Figure 4). On 2015 Aug 1, Spot A and some minor spots were captured by Sussenbach and Kivits at different points in time. The rotation of Spot A is clearly detectable.
The presence of Spot A was confirmed by several other observers (Maxson, Milika & Nicholas, Miles, Peach and Wesley; see Figure 5). There is also a visual observation of the spot by Maksymowicz (Figure 6).
Spot A remained visible till 2015 December. A second, fainter feature at latitude 20°N, Spot B, was visible in 2015 September and October (Figures 7 & 8). Due to the small size of the planet, this fainter spot was not visible under poor seeing conditions.
Drift of Spot A
Due to the stability and brightness of Spot A, this feature was followed by amateurs and measurements of its changing position allowed the establishment of its drift. The coordinates of the spot on Neptune’s disc were measured in 26 images using WinJUPOS. Accurate measurements on a tiny disc are a challenge in itself. A complicating factor is that due to limb darkening and contrast enhancement, very often the apparent disc of the planet is smaller than the actual outline, which causes very inaccurate coordinate values (Figure 9). Figure 9 clearly indicates that the Neptune image is smaller than the outline, whereas the angular distance of Triton to the centre of Neptune is equal.
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